This study is a randomized, partially double-blind, three-arm, multi-center trial in HIV-infected subjects with long-term nucleoside exposure. Eligible subjects are participants in ACTG 302 or 303 who continue on their initially assigned nucleoside therapy. Subjects with plasma HIV RNA > 500 copies/ml, confirmed either at baseline or during their monitoring phase, will be randomized to (1) 2 RTIs + Nelfinavir (NFV), (2) 2 RTIs + DMP-266, or (3) 2 RTIs + NFV + DMP-266. The study will be partially blinded in that subjects will receive open-label reverse transcriptase inhibitors. The duration of study treatment is planned to be at least 48 weeks. Subjects with plasma HIV RNA < 500 copies/ml at baseline will continue to receive their current 302/303 nucleoside regimen, taken in an open-label capacity, as long as serial determinations of HIV RNA remain < 500 copies/ml. The cohort of subjects with delayed randomization will be analyzed separately; analysis of Part A subjects will not depend on completion of the study by the Part B subjects.